In the prior toroidal transformer, disclosed in application Ser. No. 06/750,045, filed June 27, 1985, entitled "Toroidal Electrical Transformer and Method of Producing Same," this application being a continuation of 06/337,356, filed Jan. 6, 1982, and in a copending application, Ser. No. 06/662,312, filed Oct. 17, 1984, entitled "Apparatus And Method For Fabricating A Low Voltage Winding For A Toroidal Transformer" , an insulation structure surrounded the core (and bobbin if used) and served to insulate the low voltage winding from the core. This insulation was in the form of an arcuate (semi-toroidal) tube upon which the low voltage winding was wound. After the low voltage winding was wound, assembled with the high voltage winding, and placed in position with a similar assembly to form approximately 330.degree. of a toroid, a core was wound into the toroidal passage provided by the two insulating tubes.
Each insulating tube was made from electrical kraft insulating paper which was wet molded into the form of the tube. As is well known in the art, electrical kraft paper performs an insulating function by virtue of oil impregnation. Because of the excellent insulation properties of oil-impregnated electrical kraft paper, electrical kraft paper is widely used in transformer construction. To insure proper oil impregnation of the kraft paper, and thereby insure that the insulation will have the appropriate dielectric strength, transformers are typically heated and evacuated after assembly to remove the moisture and air from the kraft paper. While in such an evacuated state, oil is introduced into the transformer which occupies the microscopic spaces within the electrical kraft paper which were occupied by air before the evacuation.
In recent years, the transformer industry has made various efforts to substitute plastic materials for the successful and widely-used oil-impregnated electrical kraft paper, but only with limited success. Oil-impregnated electrical kraft paper continues to be the preferred insulation material in the oil-insulated transformer industry.
In the above-referenced applications, the molded, oil-impregnated, electrical kraft insulation paper functioned well. However, even though kraft electrical paper can be successfully molded by wetting the paper, it was an expensive process. The wet molding process, as is well known, requires a time-consuming drying step before the molded paper insulation can be used. Efforts to speed up the drying process through the application of heat have been only partially successful since very high temperatures, which would cause the water to vaporize into steam, would damage the paper. Accordingly, only relatively low temperatures can be used resulting in long drying times.
In addition to the cost disadvantage of the molded paper insulation, the electrical kraft paper tube had strength limitations relative to the heavy forces imposed upon the tube when heavy gauge conductors were wound upon the tube. This required the use of a relatively close-fitting arbor placed internally of the tube to support the walls of the tube during winding as disclosed in the pending patent application of Randall L. Schlake, et al., Ser. No. O6/698,98l,filed Feb. 6, 1985, entitled "Apparatus And Method For Fabricating A Low Voltage Winding For A Toroidal Transformer". To prevent the walls from collapsing during winding of the relatively heavy gauge conductor, it was necessary to have close dimensional correspondence between the electrical kraft paper tube and the internally-positioned arbor. That dimensional requirement complicated the manufacture of the electrical kraft paper tube since it added a manufacturing tolerance which was not easily met in a wet molded paper product.